The NRA demands that utilities evaluate the potential risks presented by any volcanoes within 160 kilometers of a given plant. That evaluation begins with a look through the written record for any mentions of eruptions plus examining the geological features of the area to determine if there is any chance the volcano will be active again in the future. If a future eruption can't be ruled out, then the utility must determine whether pyroclastic flows -- fast moving clouds of hot gas and volcanic matter -- or lava flows could reach the plant. If there is such a risk, then the plant site is labeled unsuitable and the reactors banned from going on line.

In the case of the Sendai nuclear plant's No. 1 and 2 reactors, it was found that there were five volcanoes with histories of cataclysmic eruptions within the 160-kilometer zone. A "cataclysmic eruption" is one that spews vast amounts of magma, causing large-scale ground subsidence and creating a caldera.

NRA inspectors found that there was "sufficiently little chance" of a cataclysmic eruption that could cause a pyroclastic flow to hit the Sendai plant grounds while the station was in operation. Furthermore, the regulatory body determined that measures to deal with up to 15 centimeters of volcanic ash from the Sakurajima volcano -- about 50 kilometers distant -- would be enough to maintain plant safety.

The NRA also called on utilities to make preparations to shut down reactors and move the nuclear fuel out of their nuclear plants if there was any sign of an impending cataclysmic eruption detected. Sendai plant operator Kyushu Electric Power Co. assured the NRA that the utility would spot signs of such an eruption by keeping a close watch out for changes in the Earth's crust caused by magma accumulation, and the regulator accepted this explanation.

However, some volcanologists have pointed out that it is very difficult to predict the timing or scale of a cataclysmic eruption. Furthermore, there is neither a predetermined spot to move the nuclear fuel to nor a set standard for the NRA to order reactor shutdowns.

Across Japan, the Genkai nuclear station's No. 3 and 4 reactors (which have passed NRA safety inspections ahead of a planned restart), the No. 1 to 3 reactors at Hokkaido Electric Power Co.'s Tomari nuclear plant (where the volcanic risk inspection has nearly been completed), and the No. 2 unit at the Shimane nuclear power station run by Chugoku Electric Power Co. are all close to volcanoes with calderas. However, the NRA has never declared a plant site unfit due to the threat of volcanic activity.

Regarding the Hiroshima High Court's Dec. 13 decision to order Shikoku Electric Power Co. to shut the No. 3 reactor at its Ikata power plant in Ehime Prefecture, NRA chief Toyoshi Fuketa told reporters, "I am not directly concerned (with the case) so I am not in a position to comment." He added that the ruling would have "no effect" on NRA inspections.

Hokkaido University specially appointed professor of nuclear reactor engineering Tadashi Narabayashi, meanwhile, said the court decision was the product of "logical leaps."

"Stopping (reactor) operation based on personal rights requires an imminent danger," Narabayashi wrote in a comment to the Mainichi Shimbun. "It's difficult to say that the chance of a cataclysmic eruption, which is thought to happen only about once in 10,000 years, meets that definition. The Ikata plant's No. 3 unit is protected from falling volcanic material and has an enhanced reactor core cooling system, so there is simply no probability of an incident that would endanger the lives of the people in the city of Matsuyama or Hiroshima."

Meanwhile, Kobe University magma specialist Yoshiyuki Tatsumi praised the court ruling as "based on scientific knowledge grounded in current volcanology."

"There is about a 1 percent chance of a cataclysmic eruption in Japan in the next 100 years, so mathematically speaking, one could happen at any time," he continued. "At present, we do not know what kinds of signs would portend such an eruption. It is also unknown how much magma has built up under Mount Aso (in Kumamoto Prefecture), so the government needs to strengthen its observations there among other measures."